The present invention relates to the magnetic resonance arts. It finds particular application in conjunction with medical diagnostic magnetic resonance imaging and will be described with particular reference thereto. However, it is to be appreciated that the present invention is also amenable to magnetic resonance spectroscopy and magnetic resonance imaging for other applications.
Generally, nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI) techniques employ a spatially uniform and temporally constant main magnetic field, B.sub.0, generated through an examination region in which a patient or subject being examined is placed. Conventionally, the uniform main magnetic field is generated in one of two ways. The first method employs a cylindrically shaped solenoidal main magnet. The central bore of the main magnetic defines the examination region in which a horizontally direct main magnetic field is generated. The second method employs a main magnet having an open geometry. Typically the main magnet has opposing pole pieces arranged facing one another to define the examination region therebetween. The pole pieces are connected by a ferrous flux return path. This configuration generates a substantially uniform vertical main magnetic field within the examination region. Open geometry main magnets have been able to solve important MRI issues such as, increasing patient aperture, avoiding patient claustrophobia, and improving access for interventional MRI applications.
A number of interventional MRI procedures such as biopsies, laser ablation, and other procedures are done on both the horizontal bore cylindrical type magnet systems as well as the vertical field open geometry magnet systems. However, the range of procedures is limited due to the presence of the cryostat, poles, and/or other magnet structures surrounding the patient. This limits access to the patient for surgery and determines such things as the available patient positions, types and size of equipment that may be employed, and the like. Consequently, many MRI interventional procedures are done staged. Staged procedures involve imaging the patient and then moving the patient to a different location and/or position for surgery. Additional images are obtained by moving the patient back into the imaging region. However, staged procedures are accompanied by certain inherent drawbacks. For example, in many types of surgery patient movement is unwanted and in this regard staged procedures are unsuitable. Furthermore, subsequent images may not properly align with previous images as repositioning the patient in the same exact location within the examination region is extremely difficult. As the procedure progresses, this may lead to faulty predictions regarding the location of a patient's anatomy.
In cases of emergency or complications during surgery, a high degree of patient access is desirable. However, the horizontal bore cylindrical magnet systems lack sufficient access in this regard. While the open geometry magnet systems provide improved access over the cylindrical magnet systems, in certain emergency circumstances this degree of access is still too limited.
More recently, many surgical procedures are done using a minimally invasive approach. Such an approach is generally easier on the patient, promotes the quickest recovery, and is often the least expensive. Some types of minimally invasive surgery have been facilitated through the use of image guided surgery techniques. High patient access in these types of surgery is extremely advantageous from several perspectives. First, it is necessary for the surgeon to have access to the surgery site. Second, there are preferred patient positions for the different surgeries. Finally, there are a variety of patient support items (tubes, wires, and the like) which are employed during the surgery and take up a certain amount of space. Furthermore, it is desirable that the entire surgery team which may include a number of surgeons and/or support personnel have access to the patient in order to properly fulfill their role in the surgery. However, generally speaking the MRI systems mentioned above do not provide the requisite access to achieve these goals.
The present invention contemplates a new and improved magnetic resonance imaging apparatus which overcomes the above-referenced problems and others.